This invention relates to an article having high emissivity and hence low reflectance in the thermal infra-red wavebands of typically between 6 and 30 microns, and also in the visible wavebands.
Such an article can suitably be employed in, for example, man-made space satellites where it is desirable that such satellites be maintained at the lowest possible operating temperature or within a required temperature range. In such circumstances, the article is typically used either to cover individual solar-cells which provide the electrical power by which the satellite operates or as thermal control mirrors which help prevent heat build-up of the satellite which may be due to solar heating or thermal emission from within the spacecraft itself.
Such an article is known as a cover slip if it is required for covering a solar-cell, the main purposes of which are to protect the solar-cell from ionizing radiation and micro-meteroids; and to protect the adhesive used to bond the cover slip to the solar-cell from ultra-violet radiation which would otherwise tend to degrade it. In addition to this simple protection function, the cover slip should ideally have a good emissivity in the thermal waveband regions, so that the solar-cell can be maintained at as low a temperature as possible which in turn increases the efficiency of electrical output.
To achieve improved emissivity performance in the thermal infra-red regions is difficult if the cover slip is made of glass in view of the presence of so-called reststrahlen bands which give peaks of high reflectance around the 10 and 22 micron regions respectively. This means that around these peaks of high reflectance the solar-cell cover slip or thermal control mirror, as the case may be, is not operating very efficiently as an emitter of thermal radiation which in turn means that the temperature beneath the glass substrate of the cover slip or thermal control mirror remains relatively high.
In U.S. Pat. No. 4,578,527 there is shown an article having improved reflectance suppression within one of the reststrahlen regions referred to above in which a cover-slip is provided with a high emissivity coating comprised of a series of periods, each period consisting of a spacer layer of thorium fluoride and an absorber layer of silicon dioxide. It is claimed in this patent specification, the disclosure of which is incorporated herein by reference, that the emission characteristics of the coated article used to protect a solar-cell can be increased by up to 4%. This is apparently achieved by reducing the reflectance at the reststrahlen peak which occurs around the 10 micron region, which reflectance is stated as being decreased from a peak of around 75% reflectance to a peak of below 30%. Since approximately 38% of the "black body" thermal radiation which is emitted lies in the 7 to 13 micron range, it is clear that a reduction in the reflectance by the amount indicated would result in an improvement of the overall thermal emissivity characteristics of the coating for the article on which it is used, provided that the emissivity at the other reststrahlen region is not compromised.
However, despite the foregoiong, U.S. Pat. No. 4,578,527 acknowledges that at a temperature of around 300 K 53% of the "black body" thermal radiation occurs at wavelengths greater than 13 microns, a region of the spectrum in which it is further acknowledged that refractive index data for materials such as thorium fluoride are scarce.
One significant disadvantage is using thorium fluoride as the spacer layer results from the fact that it is radioactive, being an emitter of alpha particles and therefore in the construction of any article incorporating this material due care must be taken to protect against this radiation. We have found a further and perhaps more significant disadvantage associated with the use of thorium fluoride by making careful measurements in the regions of the electromagnetic spectrum above 13 microns, which measurements show that although thorium fluoride achieves a fairly good reduction in reflectance around the 10 micron region, the reststrahlen peak occurring around the 22 micron region is actually raised from the normal 24% reflectance to 48% reflectance using this material. Since, as stated previously, 53% of the "black body" thermal radiation emitted occurs above the 13 micron region, it is clear that this enhancement rather than suppression of the reststrahlen peak around the 22 micron region will have a deleterious effect on the overall emission characteristics of the article.
In U.S. Pat. No. 4,578,527 it is stated that thorium fluoride has a very desirable characteristic in that it has a relatively low internal stress and is capable of being deposited on relatively thin substrates without causing discernible warpage.
We have discovered that dysprosium fluoride, which has a high internal stress, can nevertheless still be used as a material forming part of an article having a high emissivity coating and which gives considerably better spectral emission performance in the reststrahlen bands occurring around the 10 and 22 micron regions than the use of thorium fluoride. This discovery has arisen due to the appreciation that although dysprosium fluoride does indeed have a very high stress and therefore cannot normally be deposited in layers which are quarter-wave optical thicknesses, if deposited as a series of very thin layers sandwiched between corresponding thin layers of an absorber material, which layers in total make up the required optical thickness, then the stress effects can be reduced to an acceptable level. Surprisingly, it has been found that such a construction gives, to a first approximation, the same emission characteristics as that of a theoretical two layer design utilizing dysprosium fluoride as one of the materials, but without having the problem of stress build up which would be found in such a two-layer design.
One major advantage of using dysprosium fluoride rather than thorium fluoride is that the former gives a significantly better performance at the reststrahlen peak occurring around the 22 micron region and although in this region the reststrahlen peak is still raised by the use of the material, experiments have shown that in single layer coatings it is raised from about 24% reflectance only up to about 34% reflectance, rather than up to the 48% reflectance found to occur with the use of single layer thorium fluoride coatings. Furthermore, our experiments have indicated that the corresponding performance in the reststrahlen peak which occurs around the 10 micron region is also improved and reflectance as low as 12.5% can be achieved.